While it is possible to create public bodies individually through the admin
interface, it might be more advisable to scrape a list from a website or
crowdsource them with other people in a Google doc. You can then import these
public bodies as a CSV file into Froide.

The format of the CSV file should be like in this Google Docs Spreadsheet. You could for example copy it and either fill in public bodies collaboratively or programmatically.

The admin interface that lists public bodies has an import form at the very bottom of the page. Give a HTTP or HTTPS url of your CSV file and press the import button. The file will be downloaded and imported. Any errors will be shown to you.